Two-Layer Core Golf Ball

ABSTRACT

Golf balls consisting of a two-layer core and a cover are disclosed. The two-layer core consists of a small, relatively hard center enclosed by a relatively soft outer core layer. The specific gravity of the center and that of the outer core layer are substantially the same.

FIELD OF THE INVENTION

The present invention generally relates to golf balls, and more particularly to golf balls having two-layer cores comprising a relatively hard center surrounded by a relatively soft outer core layer.

BACKGROUND OF THE INVENTION

Golf balls having two-layer cores are known. For example, U.S. Pat. No. 5,779,562 discloses a solid golf ball comprising a solid core and a cover enclosing said core, the core comprising a center core layer and an outer core layer enclosing said center core layer, the center core layer having a specific gravity that differs from the specific gravity of the outer core layer by more than 0.1. U.S. Pat. No. 7,255,656 discloses golf balls having dual-layered cores having a relatively soft, low compression inner core surrounded by a relatively rigid outer core. Other examples of dual layer cores can be found, for example, in U.S. Pat. Nos. 4,781,383, 4,858,924, 5,002,281, 5,048,838, 5,104,126, 5,482,285, 5,490,674, 6,277,024, 6,379,269, and 6,616,550, 6,852,044, 6,916,254, and 7,125,345.

The present invention provides a novel two-layer core golf ball construction which may provide one or more of the following benefits: fine-tuning of the ball's spin rate and/or compression and higher resilience.

SUMMARY OF THE INVENTION

In one embodiment, the present invention is directed to a golf ball consisting of a core and a cover, wherein the core has an overall diameter of from 1.40 inches to 1.66 inches and consists of a center and an outer core layer. The center has a diameter of from 0.125 inches to 0.750 inches, a surface hardness of 70 Shore C or greater, and a specific gravity of from 0.50 g/cc to 1.20 g/cc. The outer core layer has a surface hardness less than that of the center and a specific gravity substantially the same as that of the center.

In another embodiment, the present invention is directed to a golf ball consisting of a core and a cover, wherein the core has an overall diameter of from 1.55 inches to 1.62 inches and consists of a center and an outer core layer. The center has a diameter of from 0.250 inches to 0.500 inches, a surface hardness of 81 Shore C or greater, and a specific gravity of from 0.50 g/cc to 1.18 g/cc. The outer core layer has a surface hardness of 90 Shore C or less and a specific gravity substantially the same as that of the center. The surface hardness of the outer core layer is less than or equal to the surface hardness of the center.

In yet another embodiment, the present invention is directed to a golf ball consisting of a core and a cover, wherein the core has an overall diameter of from 1.55 inches to 1.60 inches and consists of a center and an outer core layer. The center has a diameter of from 0.250 inches to 0.500 inches, a surface hardness of 81 Shore C or greater, and a specific gravity of from 0.50 g/cc to 1.18 g/cc. The outer core layer has a surface hardness of 90 Shore C or less and a specific gravity substantially the same as that of the center. The cover consists of an inner cover layer and an outer cover layer. The inner cover layer has a surface hardness of 65 Shore D or greater and a thickness of from 0.020 inches to 0.080 inches. The outer cover layer has a surface hardness of 60 Shore D or less and a thickness of from 0.015 inches to 0.055 inches.

DETAILED DESCRIPTION

A golf ball having a two-layer core and a cover enclosing the core is disclosed. The two-layer core consists of a center and an outer core layer. The center has a diameter within a range having a lower limit of 0.100 or 0.125 or 0.250 inches and an upper limit of 0.375 or 0.500 or 0.750 or 1.00 inches. The outer core layer encloses the center such that the two-layer core has an overall diameter within a range having a lower limit of 1.40 or 1.45 or 1.50 or 1.51 or 1.55 inches and an upper limit of 1.60 or 1.62 or 1.66 inches.

The surface hardness of the center is greater than the surface hardness of the outer core layer. Preferably, the center has a surface hardness of 70 Shore C or greater, or a surface hardness of 81 Shore C or greater, or a surface hardness of 85 Shore C or greater, or a surface hardness within a range having a lower limit of 70 or 80 Shore C and an upper limit of 90 or 95 Shore C. The surface hardness of the outer core layer is generally 80 Shore C or less, and preferably is less than 80 Shore C, or less than 70 Shore C, or less than 60 Shore C.

The surface hardness of a core is obtained from the average of a number of measurements taken from opposing hemispheres of a core, taking care to avoid making measurements on the parting line of the core or on surface defects, such as holes or protrusions. Hardness measurements are made pursuant to ASTM D-2240 “Indentation Hardness of Rubber and Plastic by Means of a Durometer.” Because of the curved surface of a core, care must be taken to insure that the core is centered under the durometer indentor before a surface hardness reading is obtained. A calibrated, digital durometer, capable of reading to 0.1 hardness units is used for all hardness measurements and is set to take hardness readings at 1 second after the maximum reading is obtained. The digital durometer must be attached to, and its foot made parallel to, the base of an automatic stand, such that the weight on the durometer and attack rate conform to ASTM D-2240.

The specific gravity of the center is less than or equal to or substantially the same as the specific gravity of the outer core layer. For purposes of the present invention, specific gravities are substantially the same if they are the same or within 0.1 g/cc of each other. Preferably, the center has a specific gravity within a range having a lower limit of 0.50 or 0.90 or 1.05 g/cc and an upper limit of 1.15 or 1.18 or 1.20 g/cc. The outer core layer preferably has a specific gravity of 1.00 g/cc or greater, or 1.05 g/cc or greater, or 1.10 g/cc or greater. In a particularly preferred embodiment, the specific gravity of the center and that of the outer core layer are substantially the same.

The center is preferably formed from a rubber composition or from a highly resilient thermoplastic polymer such as a highly neutralized polymer (“HNP”) composition. Particularly suitable thermoplastic polymers include Surlyn® ionomers, Hytrel® thermoplastic polyester elastomers, and ionomeric materials sold under the trade names DuPont® HPF 1000 and DuPont® HPF 2000, all of which are commercially available from E. I. du Pont de Nemours and Company; Iotek® ionomers, commercially available from ExxonMobil Chemical Company; and Pebax® thermoplastic polyether block amides, commercially available from Arkema Inc.

Suitable HNP compositions for use in forming the center comprise an HNP and optionally additives, fillers, and/or melt flow modifiers. Suitable HNPs are salts of homopolymers and copolymers of α,β-ethylenically unsaturated mono- or dicarboxylic acids, and combinations thereof, optionally including a softening monomer. The acid polymer is neutralized to 70% or higher, including up to 100%, with a suitable cation source. Suitable additives and fillers include, for example, blowing and foaming agents, optical brighteners, coloring agents, fluorescent agents, whitening agents, UV absorbers, light stabilizers, defoaming agents, processing aids, mica, talc, nanofillers, antioxidants, stabilizers, softening agents, fragrance components, plasticizers, impact modifiers, acid copolymer wax, surfactants; inorganic fillers, such as zinc oxide, titanium dioxide, tin oxide, calcium oxide, magnesium oxide, barium sulfate, zinc sulfate, calcium carbonate, zinc carbonate, barium carbonate, mica, talc, clay, silica, lead silicate, and the like; high specific gravity metal powder fillers, such as tungsten powder, molybdenum powder, and the like; regrind, i.e., core material that is ground and recycled; and nano-fillers. Suitable melt flow modifiers include, for example, fatty acids and salts thereof, polyamides, polyesters, polyacrylates, polyurethanes, polyethers, polyureas, polyhydric alcohols, and combinations thereof. Suitable HNP compositions also include blends of HNPs with partially neutralized ionomers as disclosed, for example, in U.S. Patent Application Publication No. 2006/0128904, the entire disclosure of which is hereby incorporated herein by reference, and blends of HNPs with additional thermoplastic and thermoset materials, including, but not limited to, ionomers, acid copolymers, engineering thermoplastics, fatty acid/salt-based highly neutralized polymers, polybutadienes, polyurethanes, polyesters, thermoplastic elastomers, and other conventional polymeric materials. Particularly suitable as a center layer material is DuPont® HPF 1000, commercially available from E. I. du Pont de Nemours and Company. Suitable HNP compositions are further disclosed, for example, in U.S. Pat. Nos. 6,653,382, 6,756,436, 6,777,472, 6,894,098, 6,919,393, and 6,953,820, the entire disclosures of which are hereby incorporated herein by reference.

Suitable rubber compositions for use in forming the center comprise a base rubber, a crosslinking agent, a filler, and a co-crosslinking or initiator agent. Typical base rubber materials include natural and synthetic rubbers, and combinations of two or more thereof. The base rubber is preferably polybutadiene or a mixture of polybutadiene with other elastomers. Particularly preferred is 1,4-polybutadiene having a cis-structure of at least 40%. More preferably, the base rubber is a high-Mooney-viscosity rubber. Lesser amounts of other thermoset materials may be incorporated into the base rubber. Such materials include, for example, cis-polyisoprene, trans-polyisoprene, balata, polychloroprene, polynorbornene, polyoctenamer, polypentenamer, butyl rubber, EPR, EPDM, styrene-butadiene, and similar thermoset materials. The crosslinking agent typically includes a metal salt, such as a zinc-, aluminum-, sodium-, lithium-, nickel-, calcium-, or magnesium-salt, of an unsaturated fatty acid or monocarboxylic acid, such as (meth) acrylic acid. Preferred crosslinking agents include zinc acrylate, zinc diacrylate (ZDA), zinc methacrylate, and zinc dimethacrylate (ZDMA), and mixtures thereof. The crosslinking agent must be present in an amount sufficient to crosslink a portion of the chains of the polymers in the resilient polymer component. The crosslinking agent is generally present in the rubber composition in an amount of from 15 to 30 phr, or from 19 to 25 phr, or from 20 to 24 phr. The desired compression may be obtained by adjusting the amount of crosslinking, which can be achieved, for example, by altering the type and amount of crosslinking agent. The initiator agent can be any known polymerization initiator which decomposes during the cure cycle, including, but not limited to, dicumyl peroxide, 1,1-di-(t-butylperoxy) 3,3,5-trimethyl cyclohexane, a-a bis-(t-butylperoxy)diisopropylbenzene, 2,5-di-(t-butylperoxy)-2,5-dimethyl hexane, di-t-butyl peroxide, n-butyl-4,4-bis(t-butylperoxy)valerate, lauryl peroxide, benzoyl peroxide, t-butyl hydroperoxide, and mixtures thereof. The rubber composition optionally contains one or more antioxidants. Antioxidants are compounds that can inhibit or prevent the oxidative degradation of the rubber. Suitable antioxidants include, for example, dihydroquinoline antioxidants, amine type antioxidants, and phenolic type antioxidants. The rubber composition may also contain one or more fillers to adjust the density and/or specific gravity of the core or cover. Fillers are typically polymeric or mineral particles. Exemplary fillers include precipitated hydrated silica, clay, talc, asbestos, glass fibers, aramid fibers, mica, calcium metasilicate, barium sulfate, zinc sulfide, lithopone, silicates, silicon carbide, diatomaceous earth, polyvinyl chloride, carbonates (e.g., calcium carbonate and magnesium carbonate), metals (e.g., titanium, tungsten, aluminum, bismuth, nickel, molybdenum, iron, lead, copper, boron, cobalt, beryllium, zinc, and tin), metal alloys (e.g., steel, brass, bronze, boron carbide whiskers, and tungsten carbide whiskers), metal oxides (e.g., zinc oxide, iron oxide, aluminum oxide, titanium oxide, magnesium oxide, and zirconium oxide), particulate carbonaceous materials (e.g., graphite, carbon black, cotton flock, natural bitumen, cellulose flock, and leather fiber), microballoons (e.g., glass and ceramic), fly ash, regrind, nanofillers and combinations thereof. The rubber composition may also contain one or more additives selected from free radical scavengers, accelerators, scorch retarders, coloring agents, fluorescent agents, chemical blowing and foaming agents, defoaming agents, stabilizers, softening agents, impact modifiers, plasticizers, and the like.

The rubber composition optionally includes a soft and fast agent. As used herein, “soft and fast agent” means any compound or a blend thereof that is capable of making a core 1) softer (have a lower compression) at a constant COR and/or 2) faster (have a higher COR at equal compression), when compared to a core equivalently prepared without a soft and fast agent. Preferably, the rubber composition contains from 0.05 phr to 10.0 phr of a soft and fast agent. In one embodiment, the soft and fast agent is present in an amount of from 0.05 phr to 3.0 phr, or from 0.05 phr to 2.0 phr, or from 0.05 phr to 1.0 phr. In another embodiment, the soft and fast agent is present in an amount of from 2.0 phr to 5.0 phr, or from 2.35 phr to 4.0 phr, or from 2.35 phr to 3.0 phr. In an alternative high concentration embodiment, the soft and fast agent is present in an amount of from 5.0 phr to 10.0 phr, or from 6.0 phr to 9.0 phr, or from 7.0 phr to 8.0 phr. In another embodiment, the soft and fast agent is present in an amount of 2.6 phr.

Suitable soft and fast agents include, but are not limited to, organosulfur or metal-containing organosulfur compounds, an organic sulfur compound, including mono, di, and polysulfides, a thiol, or mercapto compound, an inorganic sulfide compound, a Group VIA compound, a substituted or unsubstituted aromatic organic compound that does not contain sulfur or metal, an aromatic organometallic compound, or mixtures thereof. The soft and fast agent component may also be a blend of an organosulfur compound and an inorganic sulfide compound.

Suitable soft and fast agents of the present invention include, but are not limited to those having the following general formula:

where R₁-R₅ can be C₁-C₈ alkyl groups; halogen groups; thiol groups (—SH), carboxylated groups; sulfonated groups; and hydrogen; in any order; and also pentafluorothiophenol; 2-fluorothiophenol; 3-fluorothiophenol; 4-fluorothiophenol; 2,3-fluorothiophenol; 2,4-fluorothiophenol; 3,4-fluorothiophenol; 3,5-fluorothiophenol 2,3,4-fluorothiophenol; 3,4,5-fluorothiophenol; 2,3,4,5-tetrafluorothiophenol; 2,3,5,6-tetrafluorothiophenol; 4-chlorotetrafluorothiophenol; pentachlorothiophenol; 2-chlorothiophenol; 3-chlorothiophenol; 4-chlorothiophenol; 2,3-chlorothiophenol; 2,4-chlorothiophenol; 3,4-chlorothiophenol; 3,5-chlorothiophenol; 2,3,4-chlorothiophenol; 3,4,5-chlorothiophenol; 2,3,4,5-tetrachlorothiophenol; 2,3,5,6-tetrachlorothiophenol; pentabromothiophenol; 2-bromothiophenol; 3-bromothiophenol; 4-bromothiophenol; 2,3-bromothiophenol; 2,4-bromothiophenol; 3,4-bromothiophenol; 3,5-bromothiophenol; 2,3,4-bromothiophenol; 3,4,5-bromothiophenol; 2,3,4,5-tetrabromothiophenol; 2,3,5,6-tetrabromothiophenol; pentaiodothiophenol; 2-iodothiophenol; 3-iodothiophenol; 4-iodothiophenol; 2,3-iodothiophenol; 2,4-iodothiophenol; 3,4-iodothiophenol; 3,5-iodothiophenol; 2,3,4-iodothiophenol; 3,4,5-iodothiophenol; 2,3,4,5-tetraiodothiophenol; 2,3,5,6-tetraiodothiophenoland; zinc salts thereof, non-metal salts thereof, for example, ammonium salt of pentachlorothiophenol; magnesium pentachlorothiophenol; cobalt pentachlorothiophenol; and mixtures thereof. Preferably, the halogenated thiophenol compound is pentachlorothiophenol, which is commercially available in neat form or under the tradename STRUKTOL®, a clay-based carrier containing the sulfur compound pentachlorothiophenol loaded at 45 percent (correlating to 2.4 parts PCTP). STRUKTOL® is commercially available from Struktol Company of America of Stow, Ohio. PCTP is commercially available in neat form from eChinachem of San Francisco, Calif. and in the salt form from eChinachem of San Francisco, Calif. Most preferably, the halogenated thiophenol compound is the zinc salt of pentachlorothiophenol, which is commercially available from eChinachem of San Francisco, Calif. Additional examples are disclosed in U.S. Pat. No. 7,148,279, the entire disclosure of which is hereby incorporated herein by reference.

As used herein, “organosulfur compound(s)” refers to any compound containing carbon, hydrogen, and sulfur, where the sulfur is directly bonded to at least 1 carbon. As used herein, the term “sulfur compound” means a compound that is elemental sulfur, polymeric sulfur, or a combination thereof. It should be further understood that the term “elemental sulfur” refers to the ring structure of S₈ and that “polymeric sulfur” is a structure including at least one additional sulfur relative to elemental sulfur.

Additional suitable examples of soft and fast agents include, but are not limited to, 4,4′-diphenyl disulfide; 4,4′-ditolyl disulfide; 2,2′-benzamido diphenyl disulfide; bis(2-aminophenyl)disulfide; bis(4-aminophenyl)disulfide; bis(3-aminophenyl)disulfide; 2,2′-bis(4-aminonaphthyl)disulfide; 2,2′-bis(3-aminonaphthyl)disulfide; 2,2′-bis(4-aminonaphthyl)disulfide; 2,2′-bis(5-aminonaphthyl)disulfide; 2,2′-bis(6-aminonaphthyl)disulfide; 2,2′-bis(7-aminonaphthyl)disulfide; 2,2′-bis(8-aminonaphthyl)disulfide; 1,1′-bis(2-aminonaphthyl)disulfide; 1,1′-bis(3-aminonaphthyl)disulfide; 1,1′-bis(3-aminonaphthyl)disulfide; 1,1′-bis(4-aminonaphthyl)disulfide; 1,1′-bis(5-aminonaphthyl)disulfide; 1,1′-bis(6-aminonaphthyl)disulfide; 1,1′-bis(7-aminonaphthyl)disulfide; 1,1′-bis(8-aminonaphthyl)disulfide; 1,2′-diamino-1,2′-dithiodinaphthalene; 2,3′-diamino-1,2′-dithiodinaphthalene; bis(4-chlorophenyl)disulfide; bis(2-chlorophenyl)disulfide; bis(3-chlorophenyl)disulfide; bis(4-bromophenyl)disulfide; bis(2-bromophenyl)disulfide; bis(3-bromophenyl)disulfide; bis(4-fluorophenyl)disulfide; bis(4-iodophenyl)disulfide; bis(2,5-dichlorophenyl)disulfide; bis(3,5-dichlorophenyl)disulfide; bis (2,4-dichlorophenyl)disulfide; bis(2,6-dichlorophenyl)disulfide; bis(2,5-dibromophenyl)disulfide; bis(3,5-dibromophenyl)disulfide; bis(2-chloro-5-bromophenyl)disulfide; bis(2,4,6-trichlorophenyl)disulfide; bis(2,3,4,5,6-pentachlorophenyl)disulfide; bis(4-cyanophenyl)disulfide; bis(2-cyanophenyl)disulfide; bis(4-nitrophenyl)disulfide; bis(2-nitrophenyl)disulfide; 2,2′-dithiobenzoic acid ethylester; 2,2′-dithiobenzoic acid methylester; 2,2′-dithiobenzoic acid; 4,4′-dithiobenzoic acid ethylester; bis(4-acetylphenyl)disulfide; bis(2-acetylphenyl)disulfide; bis(4-formylphenyl)disulfide; bis(4-carbamoylphenyl)disulfide; 1,1′-dinaphthyl disulfide; 2,2′-dinaphthyl disulfide; 1,2′-dinaphthyl disulfide; 2,2′-bis(1-chlorodinaphthyl)disulfide; 2,2′-bis(1-bromonaphthyl)disulfide; 1,1′-bis(2-chloronaphthyl)disulfide; 2,2′-bis(1-cyanonaphthyl)disulfide; 2,2′-bis(1-acetylnaphthyl)disulfide; and the like; or a mixture thereof. Preferred organosulfur components include 4,4′-diphenyl disulfide, 4,4′-ditolyl disulfide, or 2,2′-benzamido diphenyl disulfide, or a mixture thereof. A preferred organosulfur component includes 4,4′-ditolyl disulfide.

In another embodiment, metal-containing organosulfur components can be used according to the invention. Suitable metal-containing organosulfur components include, but are not limited to, cadmium, copper, lead, and tellurium analogs of diethyldithiocarbamate, diamyldithiocarbamate, and dimethyldithiocarbamate, or mixtures thereof. Additional examples are disclosed in U.S. Pat. No. 7,005,479, the entire disclosure of which is hereby incorporated herein by reference.

Suitable substituted or unsubstituted aromatic organic components that do not include sulfur or a metal include, but are not limited to, 4,4′-diphenyl acetylene, azobenzene, or a mixture thereof. The aromatic organic group preferably ranges in size from C₆ to C₂₀, and more preferably from C₆ to C₁₀. Suitable inorganic sulfide components include, but are not limited to titanium sulfide, manganese sulfide, and sulfide analogs of iron, calcium, cobalt, molybdenum, tungsten, copper, selenium, yttrium, zinc, tin, and bismuth.

A substituted or unsubstituted aromatic organic compound is also suitable as a soft and fast agent. Suitable substituted or unsubstituted aromatic organic components include, but are not limited to, components having the formula (R₁)_(x)—R₃-M-R₄—(R₂)_(y), wherein R₁ and R₂ are each hydrogen or a substituted or unsubstituted C₁₋₂₀ linear, branched, or cyclic alkyl, alkoxy, or alkylthio group, or a single, multiple, or fused ring C₆ to C₂₄ aromatic group; x and y are each an integer from 0 to 5; R₃ and R₄ are each selected from a single, multiple, or fused ring C₆ to C₂₄ aromatic group; and M includes an azo group or a metal component. R₃ and R₄ are each preferably selected from a C₆ to C₁₀ aromatic group, more preferably selected from phenyl, benzyl, naphthyl, benzamido, and benzothiazyl. R₁ and R₂ are each preferably selected from a substituted or unsubstituted C₁₋₁₀ linear, branched, or cyclic alkyl, alkoxy, or alkylthio group or a C₆ to C₁₀ aromatic group. When R₁, R₂, R₃, or R₄, are substituted, the substitution may include one or more of the following substituent groups: hydroxy and metal salts thereof; mercapto and metal salts thereof; halogen; amino, nitro, cyano, and amido; carboxyl including esters, acids, and metal salts thereof; silyl; acrylates and metal salts thereof; sulfonyl or sulfonamide; and phosphates and phosphites. When M is a metal component, it may be any suitable elemental metal available to those of ordinary skill in the art. Typically, the metal will be a transition metal, although preferably it is tellurium or selenium. In one embodiment, the aromatic organic compound is substantially free of metal, while in another embodiment the aromatic organic compound is completely free of metal.

The soft and fast agent can also include a Group VIA component. Elemental sulfur and polymeric sulfur are commercially available from Elastochem, Inc. of Chardon, Ohio. Exemplary sulfur catalyst compounds include PB(RM-S)-80 elemental sulfur and PB(CRST)-65 polymeric sulfur, each of which is available from Elastochem, Inc. An exemplary tellurium catalyst under the tradename TELLOY® and an exemplary selenium catalyst under the tradename VANDEX® are each commercially available from RT Vanderbilt.

Other suitable soft and fast agents include, but are not limited to, hydroquinones, benzoquinones, quinhydrones, catechols, and resorcinols. Suitable hydroquinones are further disclosed, for example, in U.S. Patent Application Publication No. 2007/0213440. Suitable benzoquinones are further disclosed, for example, in U.S. Patent Application Publication No. 2007/0213442. Suitable quinhydrones are further disclosed, for example, in U.S. Patent Application Publication No. 2007/0213441. Suitable catechols and resorcinols are further disclosed, for example, in U.S. Patent Application Publication No. 2007/0213144. The entire disclosure of each of these references is hereby incorporated herein by reference.

Examples of commercially available polybutadienes suitable for use in forming the center include, but are not limited to, Buna CB 23, commercially available from LANXESS Corporation; SE BR-1220, commercially available from The Dow Chemical Company; Europrene® NEOCIS® BR 40 and BR 60, commercially available from Polimeri Europa; UBEPOL-BR® rubbers, commercially available from UBE Industries, Ltd.; and BR 01 commercially available from Japan Synthetic Rubber Co., Ltd.

Suitable types and amounts of base rubber, crosslinking agent, filler, co-crosslinking agent, initiator agent and additives are more fully described in, for example, U.S. Patent Application Publication Nos. 2004/0214661, 2003/0144087, and 2003/0225197, and U.S. Pat. Nos. 6,566,483, 6,695,718, and 6,939,907, the entire disclosures of which are hereby incorporated herein by reference.

The center can also be formed from a low deformation material selected from metal, rigid plastics, polymers reinforced with high strength organic or inorganic fillers or fibers, and blends and composites thereof. Suitable low deformation materials also include those disclosed in U.S. Patent Application Publication No. 2005/0250600, the entire disclosure of which is hereby incorporated herein by reference.

The center may also comprise thermosetting or thermoplastic materials such as polyurethane, polyurea, partially or fully neutralized ionomers, thermosetting polydiene rubber such as polybutadiene, polyisoprene, ethylene propylene diene monomer rubber, ethylene propylene rubber, natural rubber, balata, butyl rubber, halobutyl rubber, styrene butadiene rubber or any styrenic block copolymer such as styrene ethylene butadiene styrene rubber, etc., metallocene or other single site catalyzed polyolefin, polyurethane copolymers, e.g., with silicone, as long as the material meets the desired coefficient of restitution (“COR”).

The outer core layer is generally formed from a rubber composition. Suitable rubber compositions include those disclosed above.

Additional materials suitable for forming the center and outer core layer include the core compositions disclosed in U.S. Pat. No. 7,300,364, the entire disclosure of which is hereby incorporated herein by reference. For example, suitable center and outer core materials include HNPs neutralized with organic fatty acids and salts thereof, metal cations, or a combination of both. In addition to HNPs neutralized with organic fatty acids and salts thereof, core compositions may comprise at least one rubber material having a resilience index of at least about 40. Preferably the resilience index is at least about 50. Polymers that produce resilient golf balls and, therefore, are suitable for the present invention, include but are not limited to CB23, CB22, commercially available from of Bayer Corp. of Orange, Tex., BR60, commercially available from Enichem of Italy, and 1207G, commercially available from Goodyear Corp. of Akron, Ohio. Additionally, the unvulcanized rubber, such as polybutadiene, in golf balls prepared according to the invention typically has a Mooney viscosity of between about 40 and about 80, more preferably, between about 45 and about 65, and most preferably, between about 45 and about 55. Mooney viscosity is typically measured according to ASTM-D1646.

The two-layer core is enclosed with a cover comprising one or more layers. Suitable cover layer materials include ionomer resins and blends thereof (particularly Surlyn® ionomer resins), polyurethanes, polyureas, (meth)acrylic acid, thermoplastic rubber polymers, polyethylene, and synthetic or natural vulcanized rubber, such as balata. Suitable commercially available ionomeric cover materials include, but are not limited to, Surlyn® ionomer resins and DuPont® HPF 1000 and HPF 2000, commercially available from E. I. du Pont de Nemours and Company; and Iotek® ionomers, commercially available from ExxonMobil Chemical Company.

Particularly suitable outer cover layer materials include relatively soft polyurethanes and polyureas. Preferably, the outer cover layer material has a material hardness, as measured by ASTM D2240, of 45 Shore D or less, or 40 Shore D or less, or from 25 Shore D to 40 Shore D, or from 30 Shore D to 40 Shore D. The flexural modulus of the cover, as measured by ASTM D6272-98 Procedure B, is preferably 500 psi or greater, or from 500 psi to 150,000 psi.

It should be understood that there is a fundamental difference between “material hardness” and “hardness as measured directly on a golf ball.” For purposes of the present disclosure, material hardness is measured according to ASTM D2240 and generally involves measuring the hardness of a flat “slab” or “button” formed of the material. Hardness as measured directly on a golf ball (or other spherical surface) typically results in a different hardness value. This difference in hardness values is due to several factors including, but not limited to, ball construction (i.e., core type, number of core and/or cover layers, etc.), ball (or sphere) diameter, and the material composition of adjacent layers. It should also be understood that the two measurement techniques are not linearly related and, therefore, one hardness value cannot easily be correlated to the other. The hardness values given herein for cover materials, including inner cover layer materials and outer cover layer materials, are material hardness values measured according to ASTM D2240.

Also suitable are blends of ionomers with thermoplastic elastomers. Suitable ionomeric cover materials are further disclosed, for example, in U.S. Pat. Nos. 6,653,382, 6,756,436, 6,894,098, 6,919,393, and 6,953,820, the entire disclosures of which are hereby incorporated by reference. Suitable polyurethane cover materials are further disclosed in U.S. Pat. Nos. 5,334,673, 6,506,851, and 6,756,436, the entire disclosures of which are hereby incorporated herein by reference. Suitable polyurea cover materials are further disclosed in U.S. Pat. Nos. 5,484,870 and 6,835,794, the entire disclosures of which are hereby incorporated herein by reference. Suitable polyurethane-urea hybrids are blends or copolymers comprising urethane or urea segments as disclosed in U.S. Patent Application Publication No. 2007/0117923, the entire disclosure of which is hereby incorporated herein by reference. Additional suitable cover materials are disclosed, for example, in U.S. Patent Application Publication No. 2005/0164810, U.S. Pat. No. 5,919,100, and PCT Publications WO00/23519 and WO00/29129, the entire disclosures of which are hereby incorporated herein by reference.

In a particular embodiment, the cover is a single layer preferably formed from an ionomeric composition. The single layer cover preferably has a surface hardness of 65 Shore D or less, or 60 Shore D or less, and a thickness within a range having a lower limit of 0.010 or 0.015 or 0.020 inches and an upper limit of 0.055 or 0.100 or 0.120 or 0.140 inches.

In another particular embodiment, the cover is a two-layer cover consisting of an inner cover layer and an outer cover layer. The inner cover layer is preferably formed from an ionomeric composition, and preferably has a surface hardness of 60 Shore D or greater, or 65 Shore D or greater, and a thickness within a range having a lower limit of 0.010 or 0.015 or 0.020 or 0.030 inches and an upper limit of 0.035 or 0.045 or 0.080 or 0.120 inches. The outer cover layer is preferably formed from a castable or reaction injection moldable polyurethane, polyurea, or copolymer or hybrid of polyurethane/polyurea. Such cover material is preferably thermosetting, but may be thermoplastic, and preferably has a surface hardness of from 20 to 70 Shore D, more preferably from 30 to 65 Shore D, and most preferably from 35 to 60 Shore D. The outer cover layer preferably has a thickness within a range having a lower limit of 0.010 or 0.015 or 0.025 inches and an upper limit of 0.035 or 0.040 or 0.055 or 0.080 inches.

In a particularly preferred embodiment, the present invention provides a golf ball consisting of: a two-layer core consisting of a center and an outer core layer, and a two-layer cover consisting of an inner cover layer and an outer cover layer. The center is formed from a thermoplastic or thermoset polymer composition and, in a particularly preferred embodiment, has one or more of the following properties: a diameter of 0.375 inches, a center hardness of 85 Shore C, a surface hardness of 85 Shore C, and a specific gravity of 1.15 g/cc. The center is preferably formed from a highly neutralized polymer composition, such as DuPont® HPF 1000, commercially available from E. I. du Pont de Nemours and Company, or a blend of an HNP with a partially neutralized ionomer. The outer core layer is preferably formed from a rubber composition and, in a particularly preferred embodiment, has one or more of the following properties: a thickness of 0.602 inches, a surface hardness of 80 Shore C, and a specific gravity of 1.15 g/cc. The two-layer core preferably has a compression of from 60 to 100, or a compression of from 60 to 75, or a compression of 70. The inner cover layer is preferably formed from a composition comprising a Li/Na blend of Surlyn® 7940/Surlyn® 8940 and, in a particularly preferred embodiment, has one or more of the following properties: a thickness of 0.030 inches and a surface hardness of 66 Shore D. Surlyn® 7940, an E/MAA copolymer in which the MAA acid groups have been partially neutralized with lithium ions, and Surlyn® 8940, an E/MAA copolymer in which the MAA acid groups have been partially neutralized with sodium ions, are commercially available from E. I. du Pont de Nemours and Company. The outer cover layer is preferably formed from a polyurethane or polyurea composition and, in a particularly preferred embodiment, has one or more of the following properties: a thickness of 0.030 inches and a surface hardness of 45 Shore D.

A moisture vapor barrier layer is optionally employed between the core and the cover. Moisture vapor barrier layers are further disclosed, for example, in U.S. Pat. Nos. 6,632,147, 6,932,720, 7,004,854, and 7,182,702, the entire disclosures of which are hereby incorporated herein by reference.

In addition to the materials disclosed above, any of the core or cover layers may comprise one or more of the following materials: thermoplastic elastomer, thermoset elastomer, synthetic rubber, thermoplastic vulcanizate, copolymeric ionomer, terpolymeric ionomer, polycarbonate, polyolefin, polyamide, copolymeric polyamide, polyesters, polyester-amides, polyether-amides, polyvinyl alcohols, acrylonitrile-butadiene-styrene copolymers, polyarylate, polyacrylate, polyphenylene ether, impact-modified polyphenylene ether, high impact polystyrene, diallyl phthalate polymer, metallocene-catalyzed polymers, styrene-acrylonitrile (SAN), olefin-modified SAN, acrylonitrile-styrene-acrylonitrile, styrene-maleic anhydride (S/MA) polymer, styrenic copolymer, functionalized styrenic copolymer, functionalized styrenic terpolymer, styrenic terpolymer, cellulose polymer, liquid crystal polymer (LCP), ethylene-propylene-diene rubber (EPDM), ethylene-vinyl acetate copolymer (EVA), ethylene propylene rubber (EPR), ethylene vinyl acetate, polyurea, and polysiloxane. Suitable polyamides for use as an additional material in compositions disclosed herein also include resins obtained by: (1) polycondensation of (a) a dicarboxylic acid, such as oxalic acid, adipic acid, sebacic acid, terephthalic acid, isophthalic acid or 1,4-cyclohexanedicarboxylic acid, with (b) a diamine, such as ethylenediamine, tetramethylenediamine, pentamethylenediamine, hexamethylenediamine, or decamethylenediamine, 1,4-cyclohexyldiamine or m-xylylenediamine; (2) a ring-opening polymerization of cyclic lactam, such as ε-caprolactam or ω-laurolactam; (3) polycondensation of an aminocarboxylic acid, such as 6-aminocaproic acid, 9-aminononanoic acid, 11-aminoundecanoic acid or 12-aminododecanoic acid; or (4) copolymerzation of a cyclic lactam with a dicarboxylic acid and a diamine. Specific examples of suitable polyamides include Nylon 6, Nylon 66, Nylon 610, Nylon 11, Nylon 12, copolymerized Nylon, Nylon MXD6, and Nylon 46.

Other preferred materials suitable for use as an additional material in golf ball compositions disclosed herein include Skypel polyester elastomers, commercially available from SK Chemicals of South Korea; Septon® diblock and triblock copolymers, commercially available from Kuraray Corporation of Kurashiki, Japan; and Kraton® diblock and triblock copolymers, commercially available from Kraton Polymers LLC of Houston, Tex.

Ionomers are also well suited for blending with compositions disclosed herein. Suitable ionomeric polymers include α-olefin/unsaturated carboxylic acid copolymer- or terpolymer-type ionomeric resins. Copolymeric ionomers are obtained by neutralizing at least a portion of the carboxylic groups in a copolymer of an α-olefin and an α,β-unsaturated carboxylic acid having from 3 to 8 carbon atoms, with a metal ion. Terpolymeric ionomers are obtained by neutralizing at least a portion of the carboxylic groups in a terpolymer of an α-olefin, an α,β-unsaturated carboxylic acid having from 3 to 8 carbon atoms, and an α,β-unsaturated carboxylate having from 2 to 22 carbon atoms, with a metal ion. Examples of suitable α-olefins for copolymeric and terpolymeric ionomers include ethylene, propylene, 1-butene, and 1-hexene. Examples of suitable unsaturated carboxylic acids for copolymeric and terpolymeric ionomers include acrylic, methacrylic, ethacrylic, a-chloroacrylic, crotonic, maleic, fumaric, and itaconic acid. Copolymeric and terpolymeric ionomers include ionomers having varied acid contents and degrees of acid neutralization, neutralized by monovalent or bivalent cations as disclosed herein. Examples of commercially available ionomers suitable for blending with compositions disclosed herein include Surlyn® ionomer resins, commercially available from E. I. du Pont de Nemours and Company, and Iotek® ionomers, commercially available from ExxonMobil Chemical Company.

Silicone materials are also well suited for blending with compositions disclosed herein. Suitable silicone materials include monomers, oligomers, prepolymers, and polymers, with or without adding reinforcing filler. One type of silicone material that is suitable can incorporate at least 1 alkenyl group having at least 2 carbon atoms in their molecules. Examples of these alkenyl groups include, but are not limited to, vinyl, allyl, butenyl, pentenyl, hexenyl, and decenyl. The alkenyl functionality can be located at any location of the silicone structure, including one or both terminals of the structure. The remaining (i.e., non-alkenyl) silicon-bonded organic groups in this component are independently selected from hydrocarbon or halogenated hydrocarbon groups that contain no aliphatic unsaturation. Non-limiting examples of these include: alkyl groups, such as methyl, ethyl, propyl, butyl, pentyl, and hexyl; cycloalkyl groups, such as cyclohexyl and cycloheptyl; aryl groups, such as phenyl, tolyl, and xylyl; aralkyl groups, such as benzyl and phenethyl; and halogenated alkyl groups, such as 3,3,3-trifluoropropyl and chloromethyl. Another type of suitable silicone material is one having hydrocarbon groups that lack aliphatic unsaturation. Specific examples include: trimethylsiloxy-endblocked dimethylsiloxane-methylhexenylsiloxane copolymers; dimethylhexenylsiloxy-endblocked dimethylsiloxane-methylhexenylsiloxane copolymers; trimethylsiloxy-endblocked dimethylsiloxane-methylvinylsiloxane copolymers; trimethylsiloxyl-endblocked methylphenylsiloxane-dimethylsiloxane-methylvinysiloxane copolymers; dimethylvinylsiloxy-endblocked dimethylpolysiloxanes; dimethylvinylsiloxy-endblocked dimethylsiloxane-methylvinylsiloxane copolymers; dimethylvinylsiloxy-endblocked methylphenylpolysiloxanes; dimethylvinylsiloxy-endblocked methylphenylsiloxane-dimethylsiloxane-methylvinylsiloxane copolymers; and the copolymers listed above wherein at least one group is dimethylhydroxysiloxy. Examples of commercially available silicones suitable for blending with compositions disclosed herein include Silastic® silicone rubber, commercially available from Dow Corning Corporation of Midland, Mich.; Blensil® silicone rubber, commercially available from General Electric Company of Waterford, N.Y.; and Elastosil® silicones, commercially available from Wacker Chemie AG of Germany.

Other types of copolymers can also be added to the golf ball compositions disclosed herein. For example, suitable copolymers comprising epoxy monomers include styrene-butadiene-styrene block copolymers in which the polybutadiene block contains an epoxy group, and styrene-isoprene-styrene block copolymers in which the polyisoprene block contains epoxy. Examples of commercially available epoxy functionalized copolymers include ESBS A1005, ESBS A1010, ESBS A1020, ESBS AT018, and ESBS AT019 epoxidized styrene-butadiene-styrene block copolymers, commercially available from Daicel Chemical Industries, Ltd. of Japan.

Ionomeric compositions used to form golf ball layers of the present invention can be blended with non-ionic thermoplastic resins, particularly to manipulate product properties. Examples of suitable non-ionic thermoplastic resins include, but are not limited to, polyurethane, poly-ether-ester, poly-amide-ether, polyether-urea, Pebax® thermoplastic polyether block amides commercially available from Arkema Inc., styrene-butadiene-styrene block copolymers, styrene(ethylene-butylene)-styrene block copolymers, polyamides, polyesters, polyolefins (e.g., polyethylene, polypropylene, ethylene-propylene copolymers, ethylene-(meth)acrylate, ethylene-(meth)acrylic acid, functionalized polymers with maleic anhydride grafting, epoxidation, etc., elastomers (e.g., EPDM, metallocene-catalyzed polyethylene) and ground powders of the thermoset elastomers.

Also suitable for forming the center and/or outer core are the compositions having high COR when formed into solid spheres disclosed in U.S. Patent Application Publication No. 2003/0130434 and U.S. Pat. No. 6,653,382, the entire disclosures of which are hereby incorporated herein by reference.

The present invention is not limited by any particular process for forming the golf ball layer(s). It should be understood that the layer(s) can be formed by any suitable technique, including injection molding, compression molding, casting, and reaction injection molding.

Golf balls of the present invention typically have a coefficient of restitution of 0.70 or greater, preferably 0.75 or greater, and more preferably 0.78 or greater. Golf balls of the present invention typically have a compression of 40 or greater, or a compression within a range having a lower limit of 50 or 60 and an upper limit of 100 or 120. Cured polybutadiene-based compositions suitable for use in golf balls of the present invention typically have a hardness of 15 Shore A or greater, and preferably have a hardness of from 30 Shore A to 80 Shore D, more preferably from 50 Shore A to 60 Shore D.

Golf balls of the present invention will typically have dimple coverage of 60% or greater, preferably 65% or greater, and more preferably 75% or greater.

The United States Golf Association specifications limit the minimum size of a competition golf ball to 1.680 inches. There is no specification as to the maximum diameter, and golf balls of any size can be used for recreational play. Golf balls of the present invention can have an overall diameter of any size. The preferred diameter of the present golf balls is from 1.680 inches to 1.800 inches. More preferably, the present golf balls have an overall diameter of from 1.680 inches to 1.760 inches, and even more preferably from 1.680 inches to 1.740 inches.

Golf balls of the present invention preferably have a moment of inertia (“MOI”) of 70-95 g·cm², preferably 75-93 g·cm², and more preferably 76-90 g·cm². For low MOI embodiments, the golf ball preferably has an MOI of 85 g·cm² or less, or 83 g·cm² or less. For high MOI embodiment, the golf ball preferably has an MOI of 86 g·cm² or greater, or 87 g·cm² or greater. MOI is measured on a model MOI-005-104 Moment of Inertia Instrument manufactured by Inertia Dynamics of Collinsville, Conn. The instrument is connected to a PC for communication via a COMM port and is driven by MOI Instrument Software version #1.2.

Golf ball cores of the present invention preferably have an overall compression of from 50 to 90, or from 60 to 85, or from 65 to 85.

Compression is an important factor in golf ball design. For example, the compression of the core can affect the ball's spin rate off the driver and the feel. As disclosed in Jeff Dalton's Compression by Any Other Name, Science and Golf IV, Proceedings of the World Scientific Congress of Golf (Eric Thain ed., Routledge, 2002) (“J. Dalton”), several different methods can be used to measure compression, including Atti compression, Riehle compression, load/deflection measurements at a variety of fixed loads and offsets, and effective modulus. For purposes of the present invention, “compression” refers to Atti compression and is measured according to a known procedure, using an Atti compression test device, wherein a piston is used to compress a ball against a spring. The travel of the piston is fixed and the deflection of the spring is measured. The measurement of the deflection of the spring does not begin with its contact with the ball; rather, there is an offset of approximately the first 1.25 mm (0.05 inches) of the spring's deflection. Very low stiffness cores will not cause the spring to deflect by more than 1.25 mm and therefore have a zero compression measurement. The Atti compression tester is designed to measure objects having a diameter of 42.7 mm (1.68 inches); thus, smaller objects, such as golf ball cores, must be shimmed to a total height of 42.7 mm to obtain an accurate reading. Conversion from Atti compression to Riehle (cores), Riehle (balls), 100 kg deflection, 130-10 kg deflection or effective modulus can be carried out according to the formulas given in J. Dalton.

Golf ball cores of the present invention are not limited by any particular hardness for the center point of the core. In a particular embodiment, the center hardness is from 30 Shore C to 80 Shore C, or from 40 Shore C to 75 Shore C, or from 45 Shore C to 70 Shore C. In another particular embodiment, the center hardness is from 60 Shore C to 95 Shore C, or from 60 Shore C to 90 Shore C, or from 65 Shore C to 80 Shore C.

Golf ball cores of the present invention may have a zero or negative or positive hardness gradient. The hardness gradient is defined by hardness measurements made at the surface of the inner core (or outer core layer) and radially inward towards the center of the inner core, typically at 2 mm increments. For purposes of the present invention, “negative” and “positive” refer to the result of subtracting the hardness value at the innermost portion of the golf ball component from the hardness value at the outer surface of the component. For example, if the outer surface of a solid core has a lower hardness value than the center (i.e., the surface is softer than the center), the hardness gradient will be deemed a “negative” gradient. To prepare a core for hardness gradient measurements, the core is gently pressed into a hemispherical holder having an internal diameter approximately slightly smaller than the diameter of the core, such that the core is held in place in the hemispherical portion of the holder while concurrently leaving the geometric central plane of the core exposed. The core is secured in the holder by friction, such that it will not move during the cutting and grinding steps, but the friction is not so excessive that distortion of the natural shape of the core would result. The core is secured such that the parting line of the core is roughly parallel to the top of the holder. The diameter of the core is measured 90 degrees to this orientation prior to securing. A measurement is also made from the bottom of the holder to the top of the core to provide a reference point for future calculations. A rough cut is made slightly above the exposed geometric center of the core using a band saw or other appropriate cutting tool, making sure that the core does not move in the holder during this step. The remainder of the core, still in the holder, is secured to the base plate of a surface grinding machine. The exposed rough core surface is ground to a smooth, flat surface, revealing the geometric center of the core, making sure that exactly half of the original height of the core, as measured above, has been removed to within ±0.004 inches. Leaving the core in the holder, the center of the core is found with a center square and carefully marked and the hardness is measured at the center mark. Hardness measurements at any distance from the center of the core may be measured by drawing a line radially outward from the center mark, and measuring and marking the distance from the center, typically in 2 mm increments. All hardness measurements performed on a plane passing through the geometric center are performed while the core is still in the holder and without having disturbed its orientation, such that the test surface is constantly parallel to the bottom of the holder. The hardness difference from any predetermined location on the core is calculated as the average surface hardness minus the hardness at the appropriate reference point, e.g., at the center of the core for a single, solid core, such that a core surface softer than its center will have a negative hardness gradient. Hardness gradients are disclosed more fully, for example, in U.S. patent application Ser. No. 11/832,163, filed on Aug. 1, 2007, the entire disclosure of which is hereby incorporated herein by reference.

When numerical lower limits and numerical upper limits are set forth herein, it is contemplated that any combination of these values may be used.

All patents, publications, test procedures, and other references cited herein, including priority documents, are fully incorporated by reference to the extent such disclosure is not inconsistent with this invention and for all jurisdictions in which such incorporation is permitted.

While the illustrative embodiments of the invention have been described with particularity, it will be understood that various other modifications will be apparent to and can be readily made by those of ordinary skill in the art without departing from the spirit and scope of the invention. Accordingly, it is not intended that the scope of the claims appended hereto be limited to the examples and descriptions set forth herein, but rather that the claims be construed as encompassing all of the features of patentable novelty which reside in the present invention, including all features which would be treated as equivalents thereof by those of ordinary skill in the art to which the invention pertains. 

1. A golf ball consisting of a core and a cover, wherein the core has an overall diameter of from 1.40 inches to 1.66 inches and consists of: a center having a diameter of from 0.125 inches to 0.750 inches, a surface hardness of 70 Shore C or greater, and a specific gravity of from 0.50 g/cc to 1.20 g/cc; and an outer core layer having a surface hardness less than the surface hardness of the center and a specific gravity substantially the same as the specific gravity of the center.
 2. The golf ball of claim 1, wherein the center is formed from a rubber composition.
 3. The golf ball of claim 1, wherein the center is formed from a partially or fully neutralized ionomeric composition.
 4. The golf ball of claim 1, wherein the core has an overall diameter of from 1.51 inches to 1.60 inches.
 5. The golf ball of claim 1, wherein the center has a diameter of from 0.250 inches to 0.375 inches.
 6. The golf ball of claim 1, wherein the center has a specific gravity of from 1.05 g/cc to 1.18 g/cc.
 7. The golf ball of claim 1, wherein the center has a surface hardness of 85 Shore C or greater.
 8. The golf ball of claim 7, wherein the outer core layer has a surface hardness of 80 Shore C or less.
 9. The golf ball of claim 1, wherein the core has an overall compression of from 60 to
 100. 10. The golf ball of claim 1, wherein the cover consists of a single layer having a surface hardness of 60 Shore D or less and a thickness of from 0.015 inches to 0.100 inches.
 11. The golf ball of claim 1, wherein the cover consists of: an inner cover layer having a surface hardness of 65 Shore D or greater and a thickness of from 0.020 inches to 0.080 inches; and an outer cover layer having a surface hardness of 60 Shore D or less and a thickness of from 0.015 inches to 0.055 inches.
 12. The golf ball of claim 1, wherein the cover consists of: an inner cover layer formed from a partially or fully neutralized polymer composition and having a surface hardness of 65 Shore D or greater and a thickness of from 0.015 inches to 0.035 inches; and an outer cover layer formed from a polyurethane or polyurea composition and having a material hardness of 50 Shore D or less and a thickness of from 0.015 inches to 0.035 inches.
 13. A golf ball consisting of: a core having an overall diameter of from 1.55 inches to 1.62 inches and consisting of: a center having a diameter of from 0.250 inches to 0.500 inches, a surface hardness of 81 Shore C or greater, and a specific gravity of from 0.50 g/cc to 1.18 g/cc; and an outer core layer having a surface hardness of 90 Shore C or less and a specific gravity substantially the same as the specific gravity of the center; wherein the surface hardness of the outer core layer is less than or equal to the surface hardness of the center; and a cover.
 14. The golf ball of claim 13, wherein the center has a diameter of from 0.350 inches to 0.375 inches.
 15. The golf ball of claim 13, wherein the center has a surface hardness of 85 Shore C or greater.
 16. The golf ball of claim 13, wherein the outer core layer has a thickness of from 0.550 inches to 0.650 inches.
 17. The golf ball of claim 13, wherein the core has an overall compression of from 60 to
 100. 18. The golf ball of claim 13, wherein the center is formed from a rubber composition.
 19. The golf ball of claim 13, wherein the center is formed from a partially or fully neutralized ionomeric composition.
 20. A golf ball consisting of: a core having an overall diameter of from 1.55 inches to 1.60 inches and consisting of: a center having a diameter of from 0.250 inches to 0.500 inches, a surface hardness of 81 Shore C or greater, and a specific gravity of from 0.50 g/cc to 1.18 g/cc; and an outer core layer having a surface hardness of 90 Shore C or less and a specific gravity substantially the same as the specific gravity of the center; and a cover consisting of: an inner cover layer having a surface hardness of 65 Shore D or greater and a thickness of from 0.020 inches to 0.080 inches; and an outer cover layer having a surface hardness of 60 Shore D or less and a thickness of from 0.015 inches to 0.055 inches.
 21. The golf ball of claim 20, wherein the surface hardness of the outer core layer is less than the surface hardness of the center.
 22. The golf ball of claim 20, wherein the surface hardness of the center is 85 Shore C or greater and the surface hardness of the outer core layer is 80 Shore C or less. 